A motor unit-based model of muscle fatigue

Muscle fatigue is a temporary decline in the force and power capacity of skeletal muscle resulting from muscle activity. Because control of muscle is realized at the level of the motor unit (MU), it seems important to consider the physiological properties of motor units when attempting to understand and predict muscle fatigue. Therefore, we developed a phenomenological model of motor unit fatigue as a tractable means to predict muscle fatigue for a variety of tasks and to illustrate the individual contractile responses of MUs whose collective action determines the trajectory of changes in muscle force capacity during prolonged activity. An existing MU population model was used to simulate MU firing rates and isometric muscle forces and, to that model, we added fatigue-related changes in MU force, contraction time, and firing rate associated with sustained voluntary contractions. The model accurately estimated endurance times for sustained isometric contractions across a wide range of target levels. In addition, simulations were run for situations that have little experimental precedent to demonstrate the potential utility of the model to predict motor unit fatigue for more complicated, real-world applications. Moreover, the model provided insight into the complex orchestration of MU force contributions during fatigue, that would be unattainable with current experimental approaches.Author summary: Skeletal muscle fatigue reduces strength during work and play and profoundly impairs motor function in many neuromuscular disorders. Muscle is composed of groupings of fibres called motor units and these have an extensive range of characteristics from small, weak, and fatigue-resistant to large, strong, and highly fatigable. Our model tracks the fatigue of an entire population of motor units making up a muscle. The model predicted, with good fidelity, the endurance times for a wide range of tasks and provided new insights into the complex orchestration of motor unit contributions to muscle force during fatigue. The model should have wide application in the fields of ergonomics, rehabilitation and exercise to predict and better understand the nature of both motor unit and whole muscle fatigue.